This invention relates to multiple pole coupling discs of the type used in an electromagnetic coupling such as an electromagnetic clutch or brake. The coupling disc may be part of a rotary or non-rotary field or may be a rotary or non-rotary armature.
A typical electromagnetic coupling is disclosed in Silvestrini et al U.S. Pat. No. 4,187,939 and, in that particular case, the coupling is an electromagnetic clutch having a rotary armature disc made of low reluctance material such as steel and having a field with a rotary coupling disc or rotor which also is made of low reluctance material. When the coil of the field is excited, magnetic flux threads a path between the rotor and the axially opposing armature and attracts the armature into engagement with the working face of the rotor to couple the two for rotation in unison.
In the coupling disclosed in the Silvestrini et al patent, the armature is formed with a ring of angularly spaced "banana" slots while the rotor is formed with two concentric rings of angularly spaced banana slots located on opposite sides of the ring of slots in the armature. The banana slots form high reluctance air gaps causing the rotor and armature to define four magnetic poles which increase the torque of a coupling having a coil of a given diameter. By forming an additional ring of slots in each of the rotor and armature, the coupling may be constructed as a six-pole coupling with even higher torque capacity.
Until just recently, the banana slots conventionally have been stamped in the rotor and armature. Presently available stamping techniques dictate that, as a general rule, the radial width of the slots cannot be substantially less than approximately 3/4 the thickness of the disc. As a result, difficulty is encountered in stamping multiple rings of slots in a comparatively thick disc which is relatively small in diameter. In addition, stamping of the slots leaves burrs at the edges of the slots and tends to impose restrictions on the location of the slots in the disc and on the shape of the slots. It is difficult to maintain concentricity between adjacent rows of slots and it is difficult to keep all portions of the disc of a uniform thickness. The design of the disc thus tends to be dictated by tooling considerations rather than magnetic characteristics.
A significant improvement in electromagnetic coupling discs is disclosed in Booth et al U.S. Pat. No. 4,951,797. In the coupling disc of that patent, the magnetic poles are delineated by closed-end grooves in the non-working face of the disc rather than by through slots so as to avoid the manufacturing difficulties which arise in the formation of slots by stamping or by other techniques such as laser cutting. To form the grooves in the disc of the Booth et al patent, the material of the disc is displaced by a metal rolling method which requires a rotating die for supporting the disc and also requires a traversing roller for pressing the disc against the die. Tooling of this type is rather complex and specialized and is not compatible with conventional manufacturing techniques which assimilate stamping.